1. Field of the Invention
The present invention relates to an image pickup optical system, and is suitable for an image pickup apparatus, for example, such as a camera for silver-salt film, digital still camera, video camera, or the like.
2. Description of the Related Art
There has been demand that, of image pickup apparatuses such as digital still cameras, film cameras, and so forth, image pickup optical systems employed for single-lens reflex cameras have long back focus with a wide field angle.
A retrofocus-type image pickup optical system has been known as an image pickup optical system having long back focus with a wide field angle. With this retrofocus-type image pickup optical system, a lens unit having overall negative refractive power is disposed at the object side. Also, a lens unit having overall positive refractive power is disposed at the image side. According to such a lens configuration, an image pickup optical system having long back focus with a wide field angle is realized.
A retrofocus-type lens with single focal length including a first lens unit having negative refractive power and a second lens unit having positive refractive power in order from the object side to the image side has been known as a retrofocus-type image pickup optical system (see Japanese Patent Laid-Open No. 9-33801, U.S. Pat. No. 6,621,645).
Also, a zoom lens including a first lens group having negative refractive power and a second lens group having positive refractive power in order from the object side to the image side has been known as a retrofocus-type image pickup optical system (see Japanese Patent Laid-Open Nos. 2001-166206 and 2005-084971).
In general, with a retrofocus-type zoom lens, the lens configuration is asymmetrical with a wide field angle, and accordingly, chromatic aberration such as chromatic aberration of magnification or the like readily occurs frequently. A retrofocus-type zoom lens wherein the chromatic aberration at this time has been corrected by employing a diffraction optical element has been known (see Japanese Patent Laid-Open Nos. 2000-147373).
Generally, a retrofocus-type image pickup optical system is made up of an asymmetric lens configuration as a whole wherein a lens unit having negative refractive power is disposed at the object side as to the aperture, and a lens unit having positive refractive power is disposed at the image side.
Also, the absolute value of the negative refractive power of the lens unit at the object side is increased so as to ensure long back focus. Therefore, occurrence of various aberrations has been apt to increase.
In particular, with a retrofocus-type image pickup optical system, an off-axis light beam passes through a position relatively distant from the optical axis with a first lens unit having negative refractive power closest to the object side, so of various aberrations, chromatic aberration of magnification readily occurs frequently.
As a method for correcting this chromatic aberration of magnification, there has been frequently employed a method for disposing a positive lens, in which high dispersion glass is employed, in a first lens unit having negative refractive power, and nearby lens units. However, when attempting to correct chromatic aberration of magnification only by this positive lens, chromatic aberration of magnification in a portion having a high image height is excessively corrected. Therefore, with many retrofocus-type image pickup optical systems, in order to balance chromatic aberration of magnification in the whole screen, correction is performed so as to have undercorrection at intermediate image height and have overcorrection at the maximum image height.
On the other hand, employing a diffraction optical element in a part of the image pickup optical system enables correction of chromatic aberration to be readily performed, and an image pickup optical system having high optical performance can be readily obtained. However, even if a diffraction optical element is provided in the lens system, it is difficult to obtain an image pickup optical system having high optical performance whereby chromatic aberration can be appropriately corrected, unless the position and power thereof and the lens configuration of the overall lens system including the diffraction optical element, and so forth, are set appropriately. Also, increasing the power of a diffraction optical part making up the diffraction optical element makes the pitch of the grating portions of diffraction gratings fine.
In order to perform correction of chromatic aberration entirely by the diffraction optical element, there is a need to increase the power of the diffraction optical element. In general, it is very difficult to manufacture with precision diffraction optical elements of which the pitch of the grating portions of the diffraction gratings is fine. Therefore, in the event of correcting chromatic aberration by employing the diffraction optical element, it becomes important to appropriately set the power of the diffraction optical part, the position where the diffraction optical part is provided, the lens configuration, and so forth.
In particular, with a retrofocus-type image pickup optical system, unless these factors are set appropriately, it becomes difficult to suitably correct various aberrations such as chromatic aberration and spherical aberration to obtain high optical performance.